1. Field of the invention
The present invention relates to a method for fabricating an SOI substrate, which is made by bonding an active substrate constituted by a second semiconductor wafer to a supporting substrate constituted by a first semiconductor wafer.
2. Description of prior art
Recently, the rapid development of integrated circuits has required improvement in the quality of fabricating SOI substrates. Particularly, in order to increase the operating speed of integrated circuits, the active substrate of an SOI substrate has to be made as thin as possible.
A prior-art method for fabricating an SOI substrate by bonding an active substrate constituted by a second semiconductor wafer to a supporting substrate constituted by a first semiconductor wafer comprises the steps of:
(1) forming an oxide film on the surface of the supporting substrate constituted by the first semiconductor wafer, bonding the active substrate constituted by the second semiconductor wafer onto the surface of the supporting substrate, and also forming an oxide film on the surface of the active substrate to form a bonded-wafer; PA1 (2) removing the unbonded portion on the circumference of the bonded-wafer by grinding or etching; PA1 (3) flat grinding to thin the active substrate; and PA1 (4) attaching a reverse side of the supporting substrate to an attaching block, and polishing to further thin the active substrate.
The thickness of the active substrate being thinned by such polishing is about 2-3 .mu.m. However, in an SOI substrate used for fabricating high-speed integrated circuit, the thickness of an active substrate should be about 0.1 .mu.m, which can be achieved by PACE processing. That is, PACE processing can be performed on the active substrate which has polished as above to form an uniform thin-film.
However, according to the method of performing PACE processing on the active substrate being attached and polished, the inner side of the supporting substrate has to be attached to the attaching block to perform polishing. Therefore, some tiny indentations and protrusions form on the active substrate if contaminations and indentations exist between the attaching block and the supporting substrate.
For example, referring to FIG. 2a, while a contamination 5 is formed on the attaching side of the attaching block 4, an indentation 5a caused by the contamination 5 is formed on the inner of the supporting substrate 1a as the attaching side is attached to the supporting substrate 1a. The indentation 5a can be formed on the upper side of the supporting substrate 1a with or without forming an oxide film 3a thereon, and the active substrate 2a is then attached to the supporting substrate 1a and is polished to form a rather thin portion 5b of the active substrate 2a as shown in FIG. 2b. When polishing is finished, the supporting substrate and active substrate are stripped off from the attaching block 4. After removing the wax 4a, the supporting substrate 1a is restored to its original shape, so that the rather thin portion 5b of the active substrate 2a forms a concave thereon as shown in FIG. 2c.
Referring to FIG. 3a, while the inner side of the supporting substrate 1b is provided with an indentation 6, by attaching the supporting substrate 1b to the attaching block 4, the portions of the oxide film 3b and the active substrate 2b corresponding to the indentation 6 are formed with indentations due to polishing pressure. Referring to FIG. 3b, the portion of the active substrate 2b becomes a rather thick portion 6a by polishing in such manner. While finished polishing, the supporting substrate and active substrate are stripped off from the attaching block 4, and the wax 4a is removed. The supporting substrate 1a is restored to its original shape, so that the rather thick portion 6a of the active substrate 2a forms a protrusion thereon as shown in FIG. 3c.
However, if there are some tiny indentations and protrusions formed on the surface of the supporting substrate, it can not be flatted even though PACE processing is performed. Therefore, an active substrate having an uniform thickness can not be obtained.